I have effected the repair of a 12-foot diameter butterfly valve having a brass seat and a brass disc edge. This valve resided in a hydroelectric facility of the Tennessee Valley Authority (TVA) and operated at what I considered to be a relatively low pressure--under 150 psi. This one repair is relevant prior art of which I am aware that is pertinent to this invention.
In this prior art repair, the following procedures were followed:
First, the valve was de-watered and moved to the open position.
Second, the valve seat attached to the valve body was machined down. A stainless steel seat was placed over the machined down seat and bolted into place. By leaving the old machined down seat in place and bolting either to the machined down portion of the seat, the valve body or both, a repair simplification occurred.
Third, the metal disc edge was removed from the valve disc.
Fourth, a rubber seal was substituted for the removed metal seat.
Finally, the valve was rotated to the closed position. Thereafter, the clamping of the rubber was used to effect excursion of the rubber to come into contact with the newly refurbished valve seat.
This repair worked. However, the reader must understand that this valve operated in what I denominate to be the low-pressure range. Further, the valve was of a dimension that the blue prints of the valve and the actual dimensions of the valve tracked fairly closely.
In what immediately follows, I describe the problem environment of this invention; this invention is applied to large high-pressure valves. In making this description I will set forth-certain difficulties. The reader is to understand that invention can reside in identifying the problem to be solved as well as the solution to the problem, once it is known.
Before describing in detail this improved repair technique, the reader should understand that the valves on which the new repair technique is proposed are extremely large. For example, one valve is 138 inches in diameter or 11 feet 6 inches in diameter. I also should make the point that flap valves in hydroelectric facilities are old and come from a time in engineering practices that are out of step with current accepted techniques and records. For example, most of these valves are at least over 30 years old and many on the order of 40 years and older. At the time of construction of these large valves, "blue prints" acted more as a guide than as a rigid design constraint to the valves. As a consequence, when such valves are repaired, reference to the drawings of the valves give the repairing engineer a rough idea of valve dimensions--but that is all. Actual valve dimensions vary widely from prints of record. For example, although the prints often indicate that the valve seats and valve seals are round, they frequently depart from what is the modern definition of the term "round." Therefore, repair techniques must be ready to accommodate unexpected dimensional excursion.
Trying to transfer my valve repair technique to large high-pressure valves left many short-comings.
First, I understood that my prior technique was not suitable pressures in excess of 150 psi. I had to locate a disc edge design with an elastic extrudable seal edge that could be relied upon not to elastically deform and then leak in high-pressure environments, such as 150 to 600 psi.
Second, the clamping of the rubber at the edge of the valve flap of the prior art design was inadequate. Such clamping was not central to the mass of the rubber edge inserted to the valve. As the present design eventually developed, finite element analysis gave the realization that clamping of the newly substituted valve seat would cause a non-symmetrical elastic extrusion of the rubber. This non-symmetrical extrusion of the rubber advanced extruded rubber having bending resistance less than the contained pressure. Such bending resistance less than the contained rubber would lead to leakage in the finished valve.
Third, while it at first seem logical to provide an increased mass of elastically extruded rubber for resistance of high pressure, it turned out that the elastic extrusion of rubber required instead a decreased mass of compressed elastically extruded rubber at the valve seat contacting portion of the flap.
Finally, in the originally fabricated edge of the valve seat, the edge protruded normally outward of the valve seat. As will hereafter become apparent, the extruded edge of the seat has angularity with respect to the seat edge. Specifically, the elastically extruded portion of the disc edge is inclined to and toward the high-pressure portion of the valve disc. This inclination gives the extruded portion of the valve flap an "over center" seating. For the seal produced by this invention to fail, the elastically extruded portion of the valve disc edge must compress with respect to the valve seat. Further, and during such compression with the valve seat, the elastically extruded portion of the valve seat must deform in a manner, which is "over-center." Thus, resistance of leakage at high pressure is enhanced by the disclosure of the disc edge design here set forth.